[PATCH net] vsock/virtio: cap TX credit to local buffer size

[Melbin K Mathew <mlbnkm1@gmail.com>]

The virtio vsock transport currently derives its TX credit directly
from peer_buf_alloc, which is set from the remote endpoint's
SO_VM_SOCKETS_BUFFER_SIZE value.

On the host side this means that the amount of data we are willing to
queue for a connection is scaled by a guest-chosen buffer size,
rather than the host's own vsock configuration. A malicious guest can
advertise a large buffer and read slowly, causing the host to allocate
a correspondingly large amount of sk_buff memory.

Introduce a small helper, virtio_transport_peer_buf_alloc(), that
returns min(peer_buf_alloc, buf_alloc), and use it wherever we consume
peer_buf_alloc:

  - virtio_transport_get_credit()
  - virtio_transport_has_space()
  - virtio_transport_seqpacket_enqueue()

This ensures the effective TX window is bounded by both the peer's
advertised buffer and our own buf_alloc (already clamped to
buffer_max_size via SO_VM_SOCKETS_BUFFER_MAX_SIZE), so a remote guest
cannot force the host to queue more data than allowed by the host's
own vsock settings.

On an unpatched Ubuntu 22.04 host (~64 GiB RAM), running a PoC with
32 guest vsock connections advertising 2 GiB each and reading slowly
drove Slab/SUnreclaim from ~0.5 GiB to ~57 GiB and the system only
recovered after killing the QEMU process.

With this patch applied, rerunning the same PoC yields:

  Before:
    MemFree:        ~61.6 GiB
    MemAvailable:   ~62.3 GiB
    Slab:           ~142 MiB
    SUnreclaim:     ~117 MiB

  After 32 high-credit connections:
    MemFree:        ~61.5 GiB
    MemAvailable:   ~62.3 GiB
    Slab:           ~178 MiB
    SUnreclaim:     ~152 MiB

i.e. only ~35 MiB increase in Slab/SUnreclaim, no host OOM, and the
guest remains responsive.

Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
Reported-by: Melbin K Mathew <mlbnkm1@gmail.com>
Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
---
 net/vmw_vsock/virtio_transport_common.c | 27 ++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index dcc8a1d58..f5afedf01 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -491,6 +491,25 @@ void virtio_transport_consume_skb_sent(struct sk_buff *skb, bool consume)
 }
 EXPORT_SYMBOL_GPL(virtio_transport_consume_skb_sent);
 
+/*
+ * Return the effective peer buffer size for TX credit computation.
+ *
+ * The peer advertises its receive buffer via peer_buf_alloc, but we
+ * cap that to our local buf_alloc (derived from
+ * SO_VM_SOCKETS_BUFFER_SIZE and already clamped to buffer_max_size)
+ * so that a remote endpoint cannot force us to queue more data than
+ * our own configuration allows.
+ */
+static u32 virtio_transport_peer_buf_alloc(struct virtio_vsock_sock *vvs)
+{
+	u32 peer  = vvs->peer_buf_alloc;
+	u32 local = vvs->buf_alloc;
+
+	if (peer > local)
+		return local;
+	return peer;
+}
+
 u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
 {
 	u32 ret;
@@ -499,7 +518,8 @@ u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
 		return 0;
 
 	spin_lock_bh(&vvs->tx_lock);
-	ret = vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
+	ret = virtio_transport_peer_buf_alloc(vvs) -
+             (vvs->tx_cnt - vvs->peer_fwd_cnt);
 	if (ret > credit)
 		ret = credit;
 	vvs->tx_cnt += ret;
@@ -831,7 +851,7 @@ virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk,
 
 	spin_lock_bh(&vvs->tx_lock);
 
-	if (len > vvs->peer_buf_alloc) {
+	if (len > virtio_transport_peer_buf_alloc(vvs)) {
 		spin_unlock_bh(&vvs->tx_lock);
 		return -EMSGSIZE;
 	}
@@ -882,7 +902,8 @@ static s64 virtio_transport_has_space(struct vsock_sock *vsk)
 	struct virtio_vsock_sock *vvs = vsk->trans;
 	s64 bytes;
 
-	bytes = (s64)vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
+	bytes = (s64)virtio_transport_peer_buf_alloc(vvs) -
+               (vvs->tx_cnt - vvs->peer_fwd_cnt);
 	if (bytes < 0)
 		bytes = 0;
 
-- 
2.34.1